An immunoassay method which uses an antigen-antibody reaction is known. According to this method, a change in the physical amount of a light caused by a color forming reaction is measured using an evanescent wave from an optical waveguide, by employing marking bodies causing a color forming reaction and a reagent. In this method, antibodies marked with the marking bodies causing a color forming reaction are used, and each marking body corresponds to each antibody. The reaction between a pair of one antigen and one antibody produces no more than a color forming reaction caused by one marking body. Thus, the color forming amount i.e. the sensitivity of detecting subject substances to be measured has a limit, and a higher sensitivity cannot be obtained easily.
On the other hand, there is known an optical waveguide type sensor chip in which fine particles are bonded to a surface of an optical waveguide through subject substances to be measured by an antigen-antibody reaction using first and second antibodies having a property of specifically bonding to the subject substances. The first antibodies are fixed to the fine particles. The second antibodies are fixed to the optical waveguide. Since the absorbency which is caused only by the fine particles bonded to a surface of the optical waveguide by the antigen antibody reaction is detected by an evanescent light near the surface of the optical waveguide, it is possible to quantify the subject substances without a step of cleaning extra specimens or secondary antibodies.
However, the fine particles may be adhered to the surface of the optical waveguide without using an antigen-antibody reaction, and the light may be also absorbed or scattered by the fine particles which are adsorbed to the surface without using an antigen-antibody reaction. As a result, a measurement error may be caused in a measurement which needs high detection sensitivity.
In any of the above methods, development of techniques for improving the sensitivity in detection of subject substances to be measured are required for an inspection item which needs to be detected with high sensitivity.